In recent years a great deal has been learned about the inhibition of smooth muscle cell growth by heparin, a phenomenon which may prove to be of major important in the inhibition of atherosclerosis. The mechanism of this effect is unknown. In other studies in this laboratory heparin has been found to enhance the action of acidic fibroblast growth factor, which is known to cause smooth muscle proliferation in vitro. We have also observed the heparin alone at very low doses could at times stimulate the growth response of fibroblasts to serum. Consequently, we thought it possible that heparin in very low doses might actually enhance smooth muscle cell growth and this is what is reported in this accompanying abstract.